The present invention is generally directed to disc drives and in particular to apparatus for centering magnetic discs in magnetic disc drives.
There are conventional disc drives which are designed to center and rotate a disc having a hub which has a first substantially square centrally located aperture and a second aperture located at a point on the hub removed from the center. The centrally located aperture receives a cylindrical motor shaft while the second aperture receives a position controlling member which provides a rotational force and a centering force to the disc. This centering force urges two walls of the central aperture into contact with the motor shaft.
In this type of conventional system the position controlling member is a driving roller. The driving roller is supported by a spring plate which is affixed to a spindle that rotates with the motor shaft and supports the disc.
In this arrangement, the positioning of the driving roller is affected by how accurately the driving roller is positioned on the spring plate, how accurately the elasticity of the spring plate is reproduced from unit to unit and the manner in which the spring plate is positioned with respect to the spindle. Therefore, the rotational and centering forces that the roller exerts on the disc vary with the positioning of the driving roller.
Since the spring plate supports the driving roller, there are a limited number of configurations for the spring plate. Generally the spring plate must be of at least a certain minimum size in order to obtain appropriate elasticity.
Another disadvantage of this conventional structure is that when the disc rotates the driving roller is subjected to a force equal and opposite to the force transmitted to the rotating disc. The spring plate is deformed until this force balances with the force of restitution of the spring plate. Since the spring plate is generally designed to flex so as to allow motion of the roller both in a direction which is perpendicular to that of the plane of the spindle and in a direction which is perpendicular to the longitudinal axis of the motor shaft, the driving roller is forced to incline backwards with respect to the direction of rotation of the disc as the spring plate is flexed.
An additional force on the disc which tends to oppose rotation of the disc is friction between the disc and magnetic heads which are used to read the disc. The friction varies from disc to disc, especially when the discs are produced by different manufacturers. Other factors which affect this force are the kind of disc, the location of the heads and environmental factors such as temperature and humidity. Thus, the degree of inclination of the rotational axis of the driving roller is subject to large variations. This is a defect of major significance in that the recording and reading of data by the heads will be unreliable or impossible because timing for recording and reading out the data is based on the position of the driving roller.
In such conventional disc drives, a small circular slippery sheet is generally attached to the center of the spindle. The disc hub is attracted to the spindle by a magnet on the spindle which urges the disc into contact with the slippery sheet. This arrangement causes the stability of the position of the disc with respect to the spindle to be unstable. The disc may float up and flap in the direction of the longitudinal axis of the motor shaft when the disc rotates.
Accordingly there is a need for a disc drive which has a more precise and reliable apparatus for centering and rotating the disc and produces results which are repeatable from disc to disc even under changes in environmental conditions, changes in position of the magnetic heads, and aging or deterioration of components of the centering apparatus.